[#779] User Guide: add an introduction (#780)
[#779] User Guide: add an introduction (#780)

Let's add an introduction to the UG which specifies the following:
* target user profile
* value proposition

Benefits:
1. the reader gets an early preview of if the app is a good fit for
   her needs
2. students peer-testing an app based on the UG has easy access to
   the target user profile and the value proposition
se-edu/addressbook-level3